The conventional gallium nitride-based light emitting diode (LED) has very low light extraction efficiency, because only about 4% of light proceeding to an air layer from the gallium nitride-based LED is extracted due to Fresnel reflection resulting from a refraction difference between a semiconductor and an air layer, and due to a total reflectivity by the law of refraction known as Snell's law.
In order to improve the low light extraction efficiency, research is being actively ongoing. As a representative method, a concavo-convex structure or a fine pattern is formed on the surface of the gallium nitride-based LED, thereby improving light extraction efficiency through a surface scattering effect or a two-dimensional (2D) photonic crystal effect. FIG. 1 is a view illustrating an example that a fine pattern is formed on the surface of a transparent electrode layer so as to improve light extraction efficiency of an LED in accordance with the conventional art.
Recent LED processes are being performed as fine nanoscale processes. As a lithography method for forming a fine pattern applicable to the fine nanoscale processes, mainly used are a holographic lithography, an electron-beam lithography, and a nanosphere lithography (NSL). However, in case of the hololithography, it is difficult to form a nanoscale pattern having a size of several tens of nanometers. And, the electron-beam lithography has a limitation in being applicable to patterning for a large area.
When compared with the hololithography and electron-beam lithography techniques, the nanosphere lithography has an advantage that the number of processes including PR coating, exposure, development, etc. can be reduced, because it does not require an additional mask when forming a nanoscale pattern. Further, polymer or silica-based beads used in the nanosphere lithography have various sizes of 30 nm˜200 μm. This can implement various patterns of various sizes including a fine nanoscale pattern.
However, the nanosphere lithography also has the following problems. Among lithography processes using such nano-sized beads, the most important process is to coat beads in a monolayer. In order to coat polymer beads in a monolayer, various process variables such as a bead weight (wt %), a spin speed, and a surfactant amount for contact with a substrate, should be calculated and changed according to the size of the polymer beads.